System for radio frequency amplification



Dec. 31, 1935.

\ Filed March 10, 1922 vemtoz Patented Dec. 31, 1935 UNITED srArssPATENT OFFEQE SYSTEM FOR RADIO FREQUENCY AMPLRFICATEON Appiication March10, 1922, Serial No. 542,525

12 Claims.

This invention relates to radio signaling and more particularly to amethod and means for receiving signals in which high frequency currentsare amplified using commercial frequency alternating or varying currentas a source of energy.

Heretofore in the art it has been necessary in the operation of radiofrequency vacuum tube amplifiers in which a reproduction of the inputvariations is desired touse a source of direct current to energize orheat the filaments and to energize the plate circuits of the vacuumtubes. Suitable dry or storage batteries have been used for this purposeand considerable inconvenience has been occasioned by the necessity ofcharging 1 and replacing the batteries.

My invention aims to eliminate the necessity of the use of suchbatteries, both for lighting the filaments and for energizing the platecircuits of the vacuum tubes by providing means whereby commercialalternating or varying currents such as the usual volt 6O cycle houselightingsupply, may be used to light the filaments and energize theplate circuits of the vacuum tubes in high frequency amplifiers. Afurther object is to provide novel means for substantially eliminatingthe low frequency current pulses in multistage ampiification by suitableselection of polarity of the secondaries or primaries of the transformers used in the diiferent stages.

Other objects are to provide novel means for maintaining a substantiallyzero difference of potential between the filament and grid of a. vacuumtube when alternating or pulsating currents are used for heating thefilaments and energizing the plate circuit, and for varyingthe'amplifier plate circuit voltage.

Other features of my apparatus will be pointed out in the followingdescription and shown in the drawing, in which Figure l is a preferredembodiment of areceiving circuit having .one stage of amplification;

Figures 2 and 3 show respectively a 4- and a 6- stage amplifier adaptedto reduce or eliminate the low frequency pulses of the current;

' Figure 4 is an explanatory diagram of the gridvoltage-plate-currentcharacteristic of the amplifier.

Referring more particularly to Figure 1, there is provided an inputcircuit I which may be inductively connected to an antenna or other waveenergy receptor in which the radio frequency voltage to be amplifiedoriginates.

A three electrode vacuum tube amplifier I, in

combination with the suitably designed radio frequency transformer T,amplifies the radio frequency input voltage and impresses it upon anoutput circuit 0. It will be seen that the vacuum tube Iand thetransformer T comprise one stage 5 of amplification. For furtheramplification similar stages are used, each comprising a vacuum tube andan output transformer. The output circuit 0 in each stage will be theinput connection for the next stage of amplification. The 10 outputcircuit for the last stage of amplification comprises a detector as isusual in the art.

The filament F-I of the vacuum tube I is heated by low frequencyalternating or pulsating current supplied by the secondary S-2 of thetransformer K, the primary P of which is connected to any suitablesource of low frequency alternating current supply which, for examplemaybe a lamp socket. The flow of current through the filament FI isregulated by a variable im- 20 pedance RI. To obtain a uni-directionalvoltage for the plate circuit of the vacuum tube I from the alternatingcurrent supply the secondary' S--3 of the transformer K is connected toany suitable rectifier such, for instance, as the 2 vacuum tuberectifier shown.

In the method shown the secondary S-3 has one of its terminals connectedto the plate and grid of a commercial type vacuum tube which areconnected together as convenient means of 30 forming a two-elementrectifier. A filament FI of vacuum tube I is heated by means of analtercuit, such as the secondary I of the oscillation 45 transformer andif desired, a battery X may be used to maintain the grid at a suitablepotential.

The filament of the vacuum tube I a is heated by a low frequencyalternating current supplied by the secondary S of the transformer K,and 50 is connected to the plate circuit of the vacuum tube I. Asuitable condenser C is shunted across one terminal of the secondary S3and the lead of the filament F 2 of the vacuum tube Iw, thus includingthe condenser C in the plate circuits of Connected across the filamentterminals 35 This 40 both tubes l and la. When so connected the tube lawill rectify the alternating voltages developed by the primary P in thesecondary 8-3 of the low frequency transformer K and cause the condenserC to accumulate a substantially uni-directional charge which may bevaried at will by manipulation of the impedance R.-2, thus varying theamplifier plate current by variation of the rectifier filament heat.

This method of obtaining a substantially direct current source ofvoltage from alternating supply will be recognized as one employing halfwave rectification. Any satisfactory method of full wave or half waverectification may be used instead of the one above described withoutdeviating from the spirit of this invention. The preferred method abovedescribed has a minimum of elements and accordingly utilizes aninexpensive apparatus and is for that reason very satisfactory.

It will be understood that the rectified current supplied to the platecircuit of the amplifier tube I is not a steady current such as thatsupplied by a battery or the like, but comprises a direct currentcomponent, upon which is overlaid a residual alternating ripple due tothe absence of perfect smoothing out circuits or filters. However, Ihave discovered that when such a current is inductively transferred fromone amplifying stage to another by means of a suitable radio frequencytransformer the effect in the output circuit, detector, or second stageis substantially eliminated.

The reason for this is that a properly designed high frequencytransformer has such a low impedance for low frequency currents that thevoltage drop across the terminals of its secondary winding ispractically negligible. Accordingly, such currents are not transferredto the secondary of the transformer and comparatively large variationsin the low frequency supply can be used without detrimental effect inthe telephones. The amplifier tubes selected should be of such acharacter that their amplification is inappreciably different fordifferent values of plate energizing voltage in the neighborhood ofsuitable operating values of the latter and the variations in platevoltage should not be permitted to exceed the values at which theamplifier gives substantially constant amplification. For instance,tubes are available in which the amplification is for all practicalpurposes the same at 100 volts as at volts, and no effect is thereforedetected in the receiving telephones if the radio frequency amplifierplate voltage is continually varied within these limits.

To further minimize the effect of a low frequency pulsating voltageapplied to the plate circuit of the vacuum tube l, I provide thesubstantially uniformly constituted impedance R, which is shunted acrossthe filament E -l and has its midpoint connected through the battery Xto the grid G. The midpoint connection to the impedance B may beadjusted slightly to one side or the other for the best operatingconditions. I am enabled by this arrangement to provide a zero value oflow frequency potential between the grid G and the filament F! of thevacuum tube at all times, thereby precluding all possibility ofdisturbance in the amplifier and eventually in the telephones whichmight otherwise result from variations of grid-filament potential at thesupply frequency. It will be understood that the impedance R is designedto be relatively ineffective to low frequency currents. It will als beunderstood that the battery X is not always necessary but may be used toset the grid-filament potential at the most suitable value for properfunctioning of the amplifier.

The above connection of the input circuit I to the approximate midpointof the impedance R causes one terminal of the filament F-! to be at apositive potential while the other terminal is at an equal negativepotential with respect to the midpoint of the impedance R. The grid G ofthe vacuum tube I is therefore simultaneously negative with respect toone terminal of the filament F-l and equally positive with respect tothe other. Such a condition results in the grid G which is connected tothe midpoint of the impedance R. having at all times a potential equalto the average potential of the filament F-l although a part of thefilament may be negative or positive with respect to the grid. However,that portion of the filament which is positive with respect to the gridcan cause no electrode current flow in the grid circuit, but the portionof the filament which is negative with respect to the grid may cause acurrent fiow which will result in variations of grid potential at thesupply frequency. Such voltage variation will be caused in part by dropin potential produced by the electronic current flowing through theinput circuit 1, and in part by the current flowing through one-half ofthe impedance R. The impedance of the input circuit I is necessarilyvery small at the filament supply frequency and accordingly produces butlittle variation of low frequency potential across it, and since I mayselect an impedance R. which is likewise suitably small for lowfrequencies, the total voltage variation due to the input circuit I andthe impedance R is very small for low frequencies. In actual practice Ihave found that such variations of grid potential as above described areso extremely small as to introduce no detrimental effects in the outputor detector circuit. In any case, the magnitude of the grid currentwhich fiows is extremely small on account of the characteristics of thevacuum tubes and the somewhat higher voltages which are advantageouslyand commonly used in the plate circuits of radio frequency amplifiers.Moreover, the operation of the grid at a suitable potential causes thetube to operate on the straight portion of the grid voltage-platecurrent characteristic so that small changes in the grid voltage due tothe low frequency supply do not change its ratio of amplification andconsequently do not affect the amplitude of the radio frequencycurrents. In fact, small variations of the grid voltage operate much inthe same way as small variations in the plate voltage.

The battery X may be used to provide a negative bias potential on thegrid of the amplifier and by making such bias potential equal to orgreater than the peak positive low frequency alternating potentialdeveloped across one-half of the impedance R the grid will at no timebecome positive with respect to any part of the filament, and no gridcurrent whatever will be developed, therefore low frequency variation ofgrid potential will be eliminated entirely.

To further describe the action of the grid circuit with alternating orpulsating direct current filament lighting, reference is made to Figure4 illustrating the grid-voltage plate-current characteristic of anamplifier. As shown in Figure 4 o the grid is negative with respect tothe middle 75 point of the filament by an amount equal to-Vg The gridwill, therefore, be at a potential Vg+Vf with respect to one end of thefilament and at a potential V9'V with respect to the other end of thefilament where Vf is one-half the filament' voltage. The electroniccurrent density will, therefore, vary alOng the filament and the maximumratio of the current densities at the two ends will be approximately asI1 is to I2. Now if the tube is operated on a straight portion of thegrid-voltage plate-current characteristic, this change of currentdensity, varying as it does both in space and time, will produce nochange in the total plate current. Dissymmetry in the construction ofthe tube which places one end of the filament in a dilferent geometricalrelation to the grid and plate from the geometrical relation of theother end, may produce unbalancing and cause a small alternating currentripple to flow in the plate circuit. This, however, may be taken care ofin the same manner as a variation in the plate voltage. No modulationdue to the amplifier will be produced in the output circuit so long asthe extreme voltages on the grid remain inside the limits within whichthe tube gives substantially constant amplification that is, remainwithin the limits of the straight portion of the grid voltage-platecurrent characteristic.

A further means for eliminating residual low frequency current ripplesmay be utilized in multi-stage amplification, Such a method is shown inFigures 2 and 3, which are connections for 4- and 6- stage amplifiersrespectively.

As described with reference to Figure l, the high frequency transformerT is extremely inefiicientfor currents of commercial frequencies, andaccordingly such pulses are not appreciably transferred to the outputcircuit 0. The effect, however, of any residual low frequency pulseswhich may be transferred by the transformer T can be counteracted inthesucceeding stages of the amplifier. For example, in the apparatus ofFigure 2 let us assume that in the plate circuit of the'first tube thereis a current variation, which is represented by I sin wt. This currentcauses the voltage in the grid circuit of the second tube to be equal toM iwlcosw't. This voltage on the grid of the tube causes a current inthe plate circuit of the second tube which may be represented bya2M1wICOSwt where a2 is the low frequency voltage-current amplificationof the Second tube. This current will in turn excite in the grid circuitof the third tube a voltage equal to -azM1M2 sin&tI, which will producea current in the plate circuit of the third tube equal to -d2a3MiM2wISl'I1wt, in which as is the amplification constant of the third tube.

The current ripple in the plate of the third tube, due to thealternating current source is the same as that in the first tube sinceall of the tubes are excited from the same alternating current source.It is seen, therefore, that any low frequency that comes from the firsttube is in opposition to the low frequency in the third tube, and thatthese two ripples will exactly counter-balance each other, provideda20L3M1M2w is equal to 1.

By proper adjustment, therefore, any residual low frequency hum whichpasses through the radio frequency transformers can be balanced out byopposing the first stage against the third stage and the second stagemay similarly be balanced against the fourth stage. Thus in anyamplifier with a number of tubes which is a multipleof four, anyresidual hum may by the use of proper constants be entirely balancedout.

In a G-tubeamplifier the-residual hum may be balanced out by adding theripple of the first and third tubes and balancing the sum against the 5ripple of the fifth tube. Similarly the second and fourth tubes may beadded and balanced against the sixth tube. This is effected by reversingthe polarity of the second transformer, as shown in Figure 3. 10,

Perfect balance will be produced by establishing the followingrelations:

It isalso apparent that tubes l and 5 may be added and balanced againsttube 3 or tubes 3 and S against tube I. The same combinations apply tothe evennumbered tubes. 20;;

In the case of the addingof tubes I and 5 and balancing'against tube 3no reversal of the transformers is necessary. In the case of addingtubes 3 and 5 and balancing against tube l the fourth transformer isreversed. In all of the cases the 25 reversalsdescribed also effect abalancing of the even numbered tubes.

In an. amplifier with an odd number of tubes the residual hum may bebalanced out for all the tubes except one. balance it is important tohave the polarity of the transformers properly connected, otherwise theresidual hum will be increased instead of being balanced out.

The foregoing analysis shows the balancing 357 out action for adisturbance of the form I sin wt.

The balancing action, however, is effective for any type of periodicfunction, since any such function can be decomposed into a series of theform I sin wt. I 40 It will further be understood that the variations inthe plate circuits of any of the tubes may be .in all .cases in Whichitis desirable to amplify variations of one frequency and to use in thelocal circuits current of another frequency, provided the differenceinfrequency between the two currents is sufliciently great toenable theapparatus to discriminate between them.

It will also be understood that the local source of current need not bean alternating current, but that the method and apparatus is applicableto varying currents having variations of suitable frequency and that thefrequency of the currents used in the several local circuits need not bethe same, provided their frequency is sufficiently different from thatof the input variations it is desired to amplify. My invention also neednot be'taken advantage of in full, and direct current taken frombatteries or another suitable source may be used in some of the circuitsin the place of, or in conjunction with, the alternating or varyingrectifier current described. Other modifications will doubtless beevident to those skilled in the art and I therefore do not intend tocon- In making adjustments for 30 fine myself to the precise detailsillustrated and described.

I claim as my invention:

1. In a system of radio communication a vacuum tube multi-stage radiofrequency amplifier, transformers interposed between successive stages,a source of low frequency current for energizing the tubes, thetransformers being so connected as to polarity that a low frequencydisturbance in one stage is opposedto a corresponding low frequencydisturbance in a stage separated from the stage in which the disturbanceoccurs by one 01 more stages.

2. The method of eliminating similar local disturbances of audiofrequency in the several stages of a multi-stage vacuum tube amplifierwhich comprises opposing the disturbance of one stage against that ofanother stage.

3. In a multi-stage vacuum tube radio frequency amplifier in the stagesof which similar audio frequency disturbances occur, means foreliminating the audio frequency disturbances comprising means forbalancing the disturbances of one stage against thoseof another.

4. In a multi-stage radio frequency amplifier the method of eliminatingsimilar disturbances of audio frequency in the several stages whichcomprises opposing the disturbances of one stage against those of aplurality of other stages.

5. In a multi-stage radio frequency amplifier the method of eliminatingsimilar disturbances of audio frequency present in all of the stages,which comprises opposing the disturbance of one stage against that ofthe stage an even number of stages higher than it in the order ofamplification.

6. In a multi-stage radio frequency amplifier a source of audiofrequency variations affecting each of the stages to substantially thesame extent and means for transmitting such variations through aplurality of stages without affecting their amplitude to an extentsufficient to render it impossible to balance said audio frequencyvariation occurring in two of the stages against one another tosubstantially eliminate them.

'7. A signal current amplifying system employing a plurality ofthree-electrode vacuum tubes arranged for multi-stage amplification of asignaling current, a source of current supply for energizing a circuitof each of said tubes and tending to cause periodic disturbingvariations in the plate circuit of said tubes, a coupling between theplate circuit of one of said tubes and the grid circuit of another ofsaid tubes, said coupling being so arranged that variations produced inthe grid circuit by variations in the plate circuit which is coupledthereto neutralize the variations in the plate circuit of said othertube.

8. In a signal current amplifying system employing three-electrodevacuum tubes arranged for multi-stage amplification of signallingcurrent, means for energizing each of said tubes for operation from acommon source of current energy tending to cause periodic disturbingvariations in the plate circuits thereof, and connected to produce saidvariations in like phase, and means for passing to the grid circuit ofone of said tubes some of the disturbing energy of another one of saidtubes of phase to neutralize the disturbing variations in the platecircuit of said first tube.

9. In an audio frequency signal current amplifying system employingthree-electrode vacuum tubes arranged for multi-stage amplification,means for energizing each of said tubes for operation from a commonsource of commercial alternating current energy tending to causeperiodic audio frequency disturbing variations in the plate circuitsthereof, and connected to produce said variations in like phase, meansfor transferring signal current energy and disturbing current energyfrom one tube to another, said means being arranged to transferdisturbing current energy of a preceding tube to neutralize disturbingcurrent energy of a succeeding tube.

10. In a signal current amplifying system employing three-electrodevacuum tubes arranged for multi-stage amplification, means forenergizing each of said tubes at least in part for operation from acommon source of alternating current connected to said tubes so that thedisturbing variations are produced in like phase, and means for passingenergizing energy from one of said tubes through the grid circuit ofanother one of said tubes in a phase to oppose the energizingfluctuations in the plate circuit of said other tube.

11. A signal amplifying circuit comprising in combination a plurality ofelectronic tubes each thereof having input and output circuits, saidtubes being arranged in cascade, a source of current supply forenergizing a circuit of each of said tubes and tending to cause periodicdisturbing variations in the output of the tubes, means for transferringthe disturbances in one of said stages to another thereof, said meansbeing so arranged that variations produced by said source of currentsupply in the one stage are neutralized by the variations in the otherstage of the system.

12. In a signal amplifying system, a plurality of cascaded electronictubes, means for energizing each of said tubes at least in part foroperation from a common source of uni-directional fluctuating current,the source being connected to the tubes so that the fluctuatingcomponent of the energizing current is impressed in like phase and meansfor passing the fluctuating energy component from one of said tubes toanother thereof in a phase to oppose the fluctuating component of theenergizing current in the output circuit of one of said tubes.

ALBERT S. BLA'I'I'ERMAN.

